In the manufacture of products and components of various complex shapes by molding or casting wherein a mold cavity is infilled with a molding or casting material in a liquid state and allowed to harden in the mold cavity, in most instances a separable mold is used that is made up of “core” and “cavity” half molds each having half cavities that when closed mate at a parting line in the mold to form the complete mold cavity. Even when so closed a minute separation remains in such mold cavity along the parting line between the two mold halves. This parting line gap is sometimes used as a venting space for the mold cavity, but in any event is unavoidable. Typically, the liquid molding material during molding will enter the gap at the parting line and then remain as a solidified rib-like protuberance on the finished molded or cast object after removal from the mold. Such parting line “witness marks” are often considered acceptable but unavoidable in the molding and casting art, although in some cases they are particularly or completely removed by various post-molding finishing operations such as machining, grinding, tumbling, etc. if the expense of such removal is deemed justified or necessary to meet the end use requirements for the cast or molded object.
In high pressure molding and casting in permanent molds, as is customary in plastic injection molding methods and machines, as well as in metal die casting methods and machines, the creation of parting line flash tends to be aggravated due to the pressurized injection of the liquefied molding or casting material. The mold mating faces must be machined to precision tolerances in order to ensure a tight fit of the two mold parts upon closure to thereby minimize the gap between the parts at the parting line plane. If the gap can be held down to say as little as three to five ten thousandths of an inch or less, creation of parting line flash can be eliminated or substantially reduced for most materials and at most injection pressures. However, this increases the cost of molds substantially. Reducing or eliminating parting line flash by tight fit of the mold parts at the parting line also greatly increases mold maintenance costs since tool wear and use tends to open up the parting line clearances or gap, thereby requiring that the mold parts be taken out of service and reworked in order to bring them back to original tolerance specifications.
One example of a surface area of a molded or cast part that cannot tolerate the existence of a parting line witness mark is where the part has an external or internal surface of revolution that is to become a fluid sealing surface by receiving thereon or therein one of various forms of elastomeric annular seal members, such as an O-ring. Since the sealing function of the O-ring requires that it be pressed or compressed tightly against the sealing surface of the part throughout the entire circumferential engagement between the seal and the part sealing surface, the existence of a transverse ridge on the sealing surface, such as that resulting from the presence of a parting line witness mark, cannot be tolerated. Such a transverse ridge lifts or raises the sealing surface of the O-ring radially away from the annular part sealing surface as the O-ring rides over the ridge, thereby either reducing or eliminating the ability of the O-ring or other seal member to effect a tight seal. In addition, the existence of such a parting line flash ridge often presents a sharp edge which will cut into the sealing material and also destroy its effectiveness as a seal.
In the prior art, closing down a parting line gap clearance or tolerance in the zone of the mold cavity that mold-forms the annular sealing surface has been a typical solution to this problem. However, with some materials and at some higher working pressures, material will still flash (i.e., escape from the mold cavity into the minute clearance between the mold parts at the parting line plane) even when the parting line gap is only somewhere between three and five ten thousandths of an inch. Typically, if the parting line clearance is sufficiently reduced in the part sealing zone area, in order to eliminate or substantially reduce the witness mark at the sealing surface, the adjacent or more remote areas along the parting line are allowed to have a greater clearance in order to provide sufficient venting of the mold cavity during the in-filling of the same. However, as the mold is cycled, it tends to wear and hence this original tight gap tolerance zone gradually opens up, and mold rework then becomes necessary. In many cases, the individual parts have to be subjected to the aforementioned post molding finishing operation, such as, in the case of plastic, trimming the flash mark or witness mark with a knife or a grinder. The economics of low volume runs often requires that this be a manual operation entailing significant labor costs as well as causing quality control problems in attempting to achieve a smooth finish sealing surface on the part within acceptable tolerance limits.